1. Field of the Invention
The invention provides a system, which can be built into a powered air-purifying respirator for providing an alarm when the flow of air to a hood, helmet, or facepiece drops below a predetermined value. The invention is particularly useful in powered air-purifying respirators such as are used by persons who are being exposed to noxious dust, fumes, mists or chemicals.
2. Description of the Related Art
U.S. Government specifications, as established by NIOSH in 30 CFR 11, require powered air-purifying respirators to supply a minimum airflow of breathable air of 4 cfm (115 liters/min.) for tight-fitting facepieces and 6 cfm (170 liters/min.) for loose-fitting helmets and hoods. To provide reasonable assurance of meeting the necessary specification, powered air-purifying respirators typically are designed to supply substantially higher airflow than required, thus affording a margin of safety against reduced airflow due to battery drain and filter clogging. Such effects are so gradual that the airflow could drop to a dangerously low level without this being noticeable to the user unless the respirator were equipped with a low-flow alarm.
We know of only one powered air-purifying respirator that is available with a low-flow alarm, namely, Model AP-28A of Shigematsu Works Ltd. At the outlet of its blower is a chamber containing a Pitot static sensor which controls a switch that is preset to actuate an alarm whenever the airflow drops below a predetermined level. The preset switch is inaccessibly positioned in the housing of the respirator to guard against accidental or deliberate readjustment of the switch setting. The Shigematsu respirator also has an "economy" setting that activates circuitry controlled by the Pitot static sensor to limit the power from the battery pack to the blower so that no more air is delivered than is necessary.
The alarm of the Shigematsu Model AP-28A is preadjusted to be activated when the airflow drops below 4 cfm (115 liters/minute). If a wearer were to wish to use a loose-fitting hood or helmet in place of a tight-fitting facepiece, the alarm would not be actuated until the airflow had dropped well below the minimum level deemed to be hazardous by NIOSH unless the alarm were preset at the factory for a higher flow rate. If such an adjustment were made, there should be means for distinguishing between the two respirators, such as a different color and/or shape, thus alerting users not to use a respirator with a low-flow alarm setting for a loose-fitting helmet or hood that requires a higher alarm setting.
West German Offenlegungsschrift DE No. 3032371 (laid open Mar. 3, 1982) shows a powered respirator for a loose-fitting hood. At the hood is an L-shaped tube containing a ball that is visible to the wearer's peripheral vision. The ball falls when the airflow drops, thus providing a visual alarm. We are not aware of commercial use of any such visual alarm.
Respirators for hospital patients are commonly equipped with alarms to alert an attendant if the airflow drops below a predetermined minimum. See, for example, U.S. Pat. No. 4,287,886 (Thompson).
3. Other Related Art
U.S. Pat. No. 4,476,729 (Stables et al.) shows a gas or airflow measuring device that employs a Pitot static sensor that is said to indicate more accurately the velocity of gas flowing through a duct by means of a flow nozzle 11 in the duct that reduces the cross-sectional area of the duct from 40 to 60 percent at the impact port of the Pitot static sensor. The Stables patent is cited only because the invention also involves means for reducing the cross-sectional area at the impact port of a Pitot static sensor.